Alkaline secondary batteries typified by a nickel-hydrogen battery and nonaqueous electrolyte secondary batteries typified by a lithium ion battery are widely used as power supplies for driving portable electronic equipment such as cell phones including smartphones, portable computers, PDAs, and portable music players. In addition, alkaline secondary batteries and the nonaqueous electrolyte secondary batteries are also widely used for power supplies for driving electric vehicles (EVs) and hybrid electric vehicles (HEVs, PHEVs) and in stationary storage battery systems for suppressing the variation in output power of photovoltaic generation, wind power generation, and the like, and for peak shifts in system power in order to store electric power during the night time and to use the electric power during daytime.
In particular, the batteries for EVs, HEVs, and PHEVs and for the stationary storage battery system are required to have high capacity and high output characteristics, and hence each battery is upsized and a number of batteries are connected in series or parallel when used. To address this, in these applications, prismatic secondary batteries are generally used from the viewpoint of space efficiency. A prismatic secondary battery that further needs physical strength commonly employs, as an outer body of the battery, a metal prismatic outer body having a mouth and a metal sealing plate for sealing up the mouth.
Such a prismatic secondary battery, for example, a prismatic nonaqueous electrolyte secondary battery, is produced as follows. For example, both faces of a positive electrode substrate made from, for example, a long sheet of aluminum foil, are coated with a positive electrode active material mixture containing a positive electrode active material to prepare a positive electrode sheet. Separately, both faces of a negative electrode substrate made from, for example, a long sheet of copper foil, are coated with a negative electrode active material mixture containing a negative electrode active material to prepare a negative electrode sheet.
Next, the positive electrode sheet and the negative electrode sheet are stacked interposing a separator made from, for example, a microporous polyethylene film therebetween, and the positive electrode sheet and the negative electrode sheet are spirally wound on a cylindrical winding core while insulating the positive electrode sheet and the negative electrode sheet from each other through the separator to prepare a cylindrical wound electrode assembly. Then, the cylindrical wound electrode assembly is pressed with a pressing machine to form a flat wound electrode assembly. Next, a positive electrode collector electrically connected to the positive electrode sheet is electrically connected to a positive electrode terminal that is insulated from a sealing plate, while a negative electrode collector electrically connected to the negative electrode sheet is electrically connected to a negative electrode terminal that is insulated from a sealing plate. Then, the flat wound electrode assembly is wrapped with a member having insulating characteristics and stored in a metal prismatic outer body; a mouth portion of the prismatic outer body is sealed with a sealing plate; an electrolyte is poured from a electrolyte pour hole provided on the sealing plate; and finally the electrolyte pour hole is sealed to produce the prismatic nonaqueous electrolyte secondary battery.
Such a prismatic secondary battery required to have high capacity and high output characteristics is required to have much higher safety than that of secondary batteries for portable small equipment. Especially, in the case of a nonaqueous electrolyte secondary battery that uses a material having very high reactivity, for example, as shown in US Patent Publication No. 2010/0233529 (US2010/0233529 (A1)) and U.S. Pat. No. 7,781,088 specification (U.S. Pat. No. 7,781,088 (B2)), this nonaqueous electrolyte secondary battery is equipped with a gas release valve for releasing internal pressure when the pressure in a battery outer body is increased and a current interruption mechanism for interrupting electrical connection between an external terminal and an electrode assembly in the outer body.
The metal sealing plate used for the prismatic secondary battery includes at least a mouth for attaching a positive electrode terminal, a mouth for attaching a negative electrode terminal, a gas release valve, and an electrolyte pour hole. The metal sealing plate commonly has a rectangular shape, a chamfered rectangular shape, a rounded rectangular shape, or an oval shape. The mouth for attaching a positive electrode terminal and the mouth for attaching a negative electrode terminal are arranged so that one mouth is formed near one end in the longitudinal direction of the sealing plate and the other mouth is formed near the other end, and each of the gas release valve and the electrolyte pour hole is provided between the negative electrode terminal and the positive electrode terminal on the sealing plate.
The gas release valve is required to have a uniform working pressure and thus needs to be machined with a high degree of accuracy. For example, as a material for forming a sealing plate for a prismatic nonaqueous electrolyte secondary battery, an aluminum material having a thickness of 0.5 to several mm is adopted, and such a material is machined so that a thin-wall portion of the gas release valve has a thickness of about 0.01 to 0.03 mm. In such a gas release valve, the variation in the thickness of the thin-wall portion by only 0.001 mm largely changes the working pressure. Therefore, it is important to provide a thin-wall portion having a uniform thickness. In general, the gas release valve is formed as a coining area through forging during the production of a sealing plate of a prismatic secondary battery.
However, the gas release valve has a large area in the sealing plate of a prismatic secondary battery, resulting in increasing the amount of forging. When a peripheral shape is not uniform, the large forging amount may generate a non-uniform metal flow (shock mark) around the forged area to break the flatness of the front face. The broken flatness of the front face around the gas release valve not only tilts members that are disposed on the front face of the sealing plate but also varies the working pressure of the gas release valve.
Furthermore, the sealing plate for a prismatic nonaqueous electrolyte secondary battery includes mouths for attaching a positive electrode terminal and a negative electrode terminal with an insulating member such as a gasket interposed therebetween, one mouth being formed near one end in a longitudinal direction of the sealing plate, and the other mouth being formed near the other end, and coining areas around the mouths. When an aluminum material is machined through forging to form the mouths and the coining-formed areas, the metal flow may not follow the mold punch in the coining-formed areas, resulting in a sink mark or a shear drop. Such a coining area having a large sink mark or a large shear drop around the mouth deteriorates the positioning performance of a gasket inserted into the mouth.